You don't have to if the customer chooses to receive receipt by text as shown in How to enable and customize payment receipt on Poynt terminal?
Alternately, to add a customer's mobile number on the Poynt terminal, perform the steps described on the Poynt-Textin integration page.
Once step #3 completes, customer is deemed to have completed opt-in to follow your business. Now your Text campaigns would be able to send messages to this customer.
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